1. Field of the Invention
This invention relates generally to a hydraulic system for an automatic power transmission for an automotive vehicle; in particular, it pertains to a system for circulating pressurized fluid through hydraulic circuits of an automatic power transmission in order to control operation of the transmission and associated components and to lubricate various components therein.
More specifically, the invention relates to a pressurized working fluid supply system for hydraulic circuits, which incorporates two mutually distinct fluid pressure source units for independent supply of working fluid for controlling transmission operation, on one hand, and for lubricating constituent elements of the automatic power transmission, on the other hand.
2. Description of the Related Art
Hydraulic systems for various automotive automatic power transmissions, inclusive of a continuously variable transmission, generally include lubrication circuits for supplying working fluid to various transmission components for their lubrication and to hydraulic control circuits for supplying control pressure to various transmission components in order to control transmission speed ratio, shift timing, operational state of lock-up clutches incorporated in a torque converter, etc. One typical construction of such hydraulic systems is disclosed in "Service Manual for RE4R01A-type Automatic Transmission" issued by Nissan Motor Co., Ltd, the assignee of the entire interests and title relative to the present invention. The lubrication circuit and the hydraulic control circuit disclosed in this publication are formed as a series circuit which receives pressurized working fluid from a common fluid pump.
It is well known in the art that the lubrication circuit requires working fluid at relatively low pressure and relatively high flow rate, whereas the hydraulic control circuit requires working fluid at relatively high pressure and relatively low flow rate. Due to the contradictory requirements in the lubrication circuit and the hydraulic control circuit, the conventional hydraulic system with the common fluid pump does not always achieve an optimum fluid supply efficiency. It has therefore been a common or typical practice of fluid supply for the lubrication circuit and the hydraulic control circuit in the prior art, to generate with the fluid pump a high pressure working fluid at a pressure level which is high enough for establishing the required line pressure level in the hydraulic control circuit, and to lower the pressure level of the supplied working fluid for obtaining the working fluid at the relatively low pressure and relatively high fluid flow rate which are required for the lubricating circuit. Therefore, it can be readily appreciated that the above-mentioned measure is disadvantageous in that an improved fluid pump operation efficiency cannot be achieved due to the necessity of lowering the pressure to satisfy the requirement for the lubrication circuit. This results in degradation of the operation efficiency of the entire hydraulic circuit.
In the prior art, there have been some proposals to use a dual pump system in the hydraulic systems of the automatic transmissions. For example, "Service Manual for W4A040-type Automatic Transmission" issued by Daimlar Benz A. G., discloses such a dual pump system in hydraulic circuit for an automatic transmission. In the dual pump system disclosed in this publication, an auxiliary fluid pump is associated with output shaft of the transmission so that it can be driven by the output torque of the output shaft. However, the auxiliary fluid pump in the disclosed system is provided merely to achieve a fail-safe function in the event that the primary fluid pump fails and becomes inoperative or lowers the discharge pressure. Therefore, even with the dual fluid pump system in the aforementioned prior art, it is still difficult, if not impossible, to optimize the operation efficiency of the fluid pump and the operation efficiency of the hydraulic system.